How do you determine the optimal cruise altitude for a jet transport?

Optimal cruise altitude maximizes range or minimizes fuel burn. At higher altitude: Lower density reduces drag at same lift, Engine TSFC typically improves (lower temperature), TAS increases for same Mach (faster travel), but Thrust available decreases. Optimal altitude is where the aircraft operates near L/D max at cruise Mach, thrust equals drag with adequate margin, and fuel flow rate is minimized. This typically results in step-climb cruise as fuel burns off and optimal altitude increases. Constraints include ATC clearances, contrail formation, and passenger comfort. Analysis uses performance software with atmospheric models.